In recent years, many proposals have been made for use of the clean energy hydrogen as an energy source. To produce hydrogen, steam reformation using a hydrocarbon fuel is the general practice. Further, in recent years, obtaining hydrogen from water by water splitting, in particular by thermochemical water splitting, has also been considered.
“Thermochemical water splitting” is the method of combining chemical reactions to cause the water splitting at a temperature lower than the case of direct heat decomposition of water. Specifically, for example, in thermochemical water splitting, in the following way, oxidation and reduction reactions between metal oxides differing in oxidation states are used to split water into hydrogen and oxygen (MRed-Ox means a redox material):MRed-Ox(high oxidation state)→MRed-Ox(low oxidation state)+O2  (endothermic reaction)MRed-Ox(low oxidation state)+H2O→MRed-Ox(high oxidation state)+H2  (exothermic reaction)Total reaction H2O→H2+½O2 
In such thermochemical water splitting, it has been proposed to divide the redox material into a plurality of parts, cause a reduction reaction at part of the redox material, and, during that time, cause an oxidation reaction at the other part of the redox material.
In relation to this, for example, H. Kaneko et al., “Rotary-Type Solar Reactor for Solar Hydrogen Production with Two-Step Water Splitting Process”, Energy & Fuel (2007), 21, pp. 2287-2293, propose to divide the redox material into a plurality of parts, attach these to rotary mechanisms, and cause a reduction reaction at part of the redox material and, during that time, cause an oxidation reaction at the other part of the redox material.
Further, in relation to this, for example, in the system called the “DLR system” (Deutsches Zentrum fuer Luft-und Raumfahrt), it has been proposed to divide the redox material into two parts, cause a reduction reaction at one part of the redox material, and, during that time, cause an oxidation reaction at the other part of the redox material.
Note that, in relation to the hydrogen production and storage using thermochemical water splitting, use of oxidation and reduction reactions between metal oxides having different oxidation states, oxidation and reduction reactions between metal oxides and metals, etc. is generally known, as shown in the following literature.
In the water splitting method proposed in Japanese Unexamined Patent Publication (A) No. 2001-270701, metal zinc and magnetite are reacted with water to generate hydrogen as a reaction product. Further, the iron oxides obtained by this reaction are decomposed to generate oxygen as a product of the decomposition reaction.
In the hydrogen generation method proposed in Japanese Unexamined Patent Publication (A) No. 07-267601, water is reduced by a metal to generate hydrogen, the generated metal oxides are reduced by a reducing agent which is produced by an endothermic chemical reaction, and the resulting reducing agent is newly introduced into the reaction process. The endothermic reaction for generating the reducing agent is performed by supplying solar energy thereto.
The hydrogen generation facility proposed in Japanese Unexamined Patent Publication (A) No. 07-144901 comprises a reduction reaction apparatus for reducing iron-based oxides, and a hydrogen generation apparatus for bringing active oxide or metal iron into contact with steam to generate hydrogen. The reduction reaction apparatus is provided with a cooling apparatus, while the hydrogen generation apparatus is provided with a heating apparatus.
The apparatus for producing hydrogen proposed in Japanese Unexamined Patent Publication (A) No. 2005-289680 has a reducing means for causing hydrogen and metal oxides to react with each other so as to separate them into the metal and steam, a storing means for storing the metal separated by the reducing means, and an oxidizing means for causing the steam and the metal stored in the storing means to react with each other so as to separate them into hydrogen and metal oxides.